(a) Field of the Invention
The present invention relates to a taffic rate controller in a packet switching network and, more particularly, to an improvement of a traffic rate controller for controlling the traffic rate in a packet switching network by monitoring the data cells transmitted from a transmitting station.
(b) Description of the Related Art
A transmitting station in a packet switching network generally forwards data for transmission from a transmitting node to a receiving node after converting the data for transmission into packet cell data each having a fixed length. A relay node in the packet switching network relays the packet cell data based on the address of the receiving station attached to the packet cell data. In this configuration, the data can be transmitted at the traffic rates based on the respective species of the data transmitted.
On the other hand, with the increase in the amount and diversification of the transmission data, a high-quality processing of the transmission data has been demanded, which highlights the importance of avoidance or cancellation of congestion in the data traffic. A tic rate control in the packet transmission is known as one of the measures for avoiding or canceling the congestion.
A conventional traffic rate control technique in the packet switching network is described hereinafter with reference to an available bit rate (ABR) service technique used in an asynchronous transfer mode (ATM switching system. It should be noted, however, that another traffic rate control technique may be also used in the ATM switching system.
The ABR service technique used in the ATM switching system is recommended in an ATM forum, xe2x80x9cTraffic Management Specificationxe2x80x9d, Version 4.0 R11, March 1961 (hereinafter, referred to as TM4.0). A traffic rate controller implementing the ABR service technique uses an allowed cell rate (ACR) specified at the transmitting node and resource management cells. The traffic rate controller monitors the resource management cells to detect the congestion in the traffic, thereby controlling the traffic rate on the transmitting side by implementing a closed loop control.
FIG. 1 shows a block diagram of a packet switching network implementing a typical traffic rate control technique conformed with TM4.0, which operates based on the ABR service technique, in an ATM switching system. The packet switching network includes a transmitting station 401, a receiving station 402 and an ATM switching system 403. The transmitting station 401 is coupled to the receiving station 402 through one of ATM switches 404 distributed in the ATM switching system 403.
When the transmitting station 401 is to forward data 405 for transmission toward the receiving station 402, the transmitting station 401 first stores the data 405 temporarily in the transmission buffer 406, then forwards the stored data in the form of data cells 407. The data cells 407 are transferred by the ATM switches 404 to the transmitting station 402 based on the address attached to each data cell. The receiving station 402 stores the data cells thus transferred in the receiving buffer 408 and takes out the stored data as effective, received data 409 to complete the data transmission.
In the ABR technique as described above, the data cell 407 is forwarded from the transmitting station 401 together with a forward resource management (FRM) cell 410 attached thereto. The receiving station 402, after receiving the FRM cell 410 attached to the data cell 407, sends back the FRM cell as a backward resource management (BRM) cell 411 to the transmitting station 401.
An explicit rate calculating section 412 calculates an explicit rate (ER) which represents information of congestion in the traffic installed between the transmitting station 401 and the receiving station 402. ER adding sections 413 and 414 add the explicit rate, supplied from the ER calculating section 412, to the FRM cell 410 supplied from the transmitting station 401, and to the BRM cell 411 supplied from the receiving station 402, when the FRM cell 410 and the BRM cell 411 pass the ATM switches 404.
The explicit rate thus supplied is the maximum cell rate allowed by the ATM switching system 403 to the transmitting station 401 at that time for the transmission without involving a congestion in the connection effected by the ATM switch 404. The explicit rate thus includes a congestion information in the connection in the ATM switching system 403. If the present load on the connection is light, the ATM switching system 403 allows a higher traffic rate to the transmitting station 401, resulting in a higher explicit rate. On the other hand, if the present load on the connection is heavy, the ATM switching system 403 allows a lower traffic rate to the transmitting station 401, resulting in a lower explicit rate in the transmitting station 401.
The allowed cell rate (ACR) as mentioned before is the maximum traffic rate in the transmitting station 401, whereas the explicit rate is the maximum traffic rate determined by the ATM switching system 403. The transmitting station 401 controls the transmission rate thereof by the function of the traffic rate controller with reference to the explicit rate attached to the BMR cell by the ATM switch 404. That is, the transmitting station 401 transmits the data at the transmission rate obtained either by increasing/decreasing the ACR value in the transmitting station 401 based on the received explicit rate or by setting the received explicit rate on the ACR value itself.
The transmitting station 401 also operates for a time-out processing wherein the transmitting station 401 deceases its ACR value in a case other than the congestion in the ATM switching system 403. This case occurs when FRM cells are not forwarded at a threshold time interval therebetween, which is called ACR decrease time factor (ADTF). The FRM cells are not forwarded at the threshold time interval ADTF in an idle state or a light-load state of the transmitting station 401, wherein the transmitting station 401 has a smaller number of available data cells for transmission per unit time length.
The threshold ADTF is introduced for the purpose of not applying an excessive sudden load to the network at the restart of the transmission after the transmitting station 401 stays in an idle state, for example, in view that a higher tic rate is not generally required at the restart due to the prior buffering of the data for transmission in the transmission buffer 406. Immediately after the time-out processing, the transmitting station starts for transmission of data at an initial cell rate (ICR), which is provided in the specification as an initialization of ACR for the operation of the transmitting station 401.
It is provided in the specification as the condition for the cell transmission that a FRM cell be attached with a fixed number xe2x80x9cGxe2x80x9d of data cells. That is, if there is only a small number of available data cells for transmission in the transmitting station 401, the rate of FRM cells is low The transmitting station effects initialization of ACR value at ICR based on the lower rate of FRM cells.
In the conventional traffic rate controller as described above, there is a problem in that the ACR value initialized in the transmitting station lowers the throughput of the traffic, especially in the case of a low-load state of the packet switching network, because the initial traffic rate ICR is extremely low.
There is another problem in the case of an application software specifying that a transmission be conducted at a time interval (jj) which is larger than the specified ADTF time (tt). In this case, even if the time interval is only slightly larger than the ADTF, (for example, even in the case of jj=101 for tt=100), the time-out procedure is conducted without fail to set the ICR on the ACR value, thereby starting the transmission at the low ICR value. Especially in the case of the ABR service technique, the amount of packet data changes at a large change rate, which lowers the throughput of the traffic, resulting in an inefficient transmission.
In order to avoid a congestion in the network caused by a sudden start of the transmission, some tic rate controllers have a function for transmitting packet dummy cells before the start of the data transmission, the dummy cells having no significance in terms of received data in the receiving station 402.
JP-A-7(1995)-95238 proposes a packet transmitter having such a traffic rate controller for use in a network system wherein a rapid increase in the traffic rate is not allowed. The packet transmitter first transmits packet dummy cells before the start of transmission, increasing the traffic rate of the packet dummy cells at the rate of change specified in the system to thereby obtain a sufficient traffic rate at the effective data cell transmission from the start thereof. The packet transmitter also transmits the dummy cells when an effective data cells are not supplied during a transmission.
In the conventional packet transmitter as described above, the packet transmitter forwards the dummy cells by forecasting the start of the transmission. This causes a problem in that the technique does not apply to the data transmission the start of which cannot be forecasted beforehand. In this case, the dummy cells can be only transmitted after detecting the command for transmission or the supply of data for transmission, which involves a larger time length before a sufficient high traffic rate is obtained.
In view of the above, it is an object of the present invention to provide a traffic rate controller in a packet switching system, which is capable of achieving a high traffic rate by avoiding setting of an ICR on the ACR value with a simple structure.
The present invention provides a taffic rate controller in a packet switching network comprising a transmitting station having a transmission data storage section for storing effective data cell therein and a transmission buffer for transmitting packet cells, a receiving station for receiving the packet cells, and a traffic rate controller for controlling a traffic rate in transmission between the transmitting station and the receiving station, the traffic rate controller including a data monitor for identifying each of the packet cells as a data cell or a resource management cell, a timer section for counting a time length elapsed since a last resource management cell is delivered from the transmission buffer, the timer section generating a trigger signal after counting a specified time length, a counter for counting a number of data cells delivered from the transmission buffer since the last resource management cell is delivered, a dummy cell supplier for responding to the trigger signal to supply dummy cells in number based on a count by the counter, a selector for selecting the effective data cells or the dummy cells as the packet cells to be stored in the transmission buffer based on the number of effective data cells stored in the transmission data storage section.
In accordance with the traffic rate controller of the present invention, a time-out procedure for setting an ICR on the ACR value is not conducted in the traffic rate controller during a data transmission wherein a sufficient number of data cells are not supplied to the transmitting station, resulting in a higher traffic rate and an efficient transmission.